The present invention relates to polymeric conjugates of anti-metabolites. In particular, the invention relates to polymeric conjugates of ara-C, gemcitabine and derivatives thereof and methods of preparing the same.
Ara-C (cytosine arabinose, cytarabine, 1-(xcex2-D-arabinofuranosyl)cytosine) is a pyrimidine nucleotide analog and an effective anticancer drug primarily used in the treatment of acute myelogenous leukemia and some other types of non-solid tumors. One of the chief advantages of ara-C is its ability to block the transition of cells from the G-phase to the S-phase. There are, however, a few shortcomings associated with the molecule as well. For example, ara-C undergoes rapid enzymatic deamination in plasma to form the inactive ara-U by deoxycytidine deaminase. The use of ara-C is also associated with development of resistance and severe toxic side effects caused by large cumulative doses of the drug. Moreover, it is generally thought of as being ineffective in treating solid tumors.
One attempt to address some of the drugs shortcomings has been to administer ara-C in combination with a deaminase inhibitor such as tetrahydro-uridine. Other attempts have centered around controlling deamination such as by modifying the 2 -position. Still further proposals have focused on making prodrugs of ara-C. For example, various N4-, 5xe2x80x2- and 3xe2x80x2-acyl derivatives have been proposed as possible ara-C prodrugs.
Prodrugs include chemical derivatives of a biologically-active parent compound which, upon administration, eventually liberate the parent compound in vivo. Prodrugs allow the artisan to modify the onset and/or duration of action of an agent in vivo and can modify the transportation, distribution or solubility of a drug in the body. Typical examples of prodrugs include organic phosphates or esters of alcohols or thioalcohols (thiols).
One type of ara-C prodrug is based on N4-peptidyl derivatives. See, for example, F. M. Menger et al. Bioconjugate Chem. 5, 162 (1994); Wipf, P. et al. Bioorganic and Medicinal Chemistry vol. 4, No. 10, pp 1585-1596 (1996); and Balajthy et al. J. Med. Chem. 35, pp. 3344-3349 (1992). Similarly, U.S. Pat. No. 5,641,758 discloses N4-octadecyl ara-C derivatives which have allegedly improved resistance to enzymatic deamination. Although N4-octadecyl prodrugs release the ara-C in vivo, it would be beneficial if the circulating life and activity against solid tumors could be enhanced.
Polymeric conjugation has been suggested as a means to improve the pharmacokinetic and pharmacodynamic properties of a parent drug compound. For example, U.S. Pat. No. 5,880,131 to Greenwald et al., the contents of which are incorporated herein by reference, discloses ester-based polymeric attachment to temporarily increase the circulating life of a parent drug. Relatively hydrolysis-resistant linkages between the polymer and the parent compound residue are not disclosed.
Although some polymer-ara-C conjugates have been reported, they are believed to have exhibited one or more drawbacks. One approach taken to synthesize polymeric drugs of ara-C is found in J. Cancer Research 44, 25-30, January (1984). The authors conjugated either polyglutamic acid (PLGA) or a related copolymer (PHEG) to the N4 position of ara-C via an amide linkage. The degree of loading onto the pendant groups of the copolymers, however, is unpredictable and can only be estimated. The location of the ara-C molecules on the copolymers is also random. Thus, it is not surprising that the rate of hydrolysis of the ara-C from these polymers is unpredictable and that the amount of ara-C released from the copolymers is also variable. Further, although the authors reported that these compounds had superior in vitro activity in an L1210 (leukemia) cell line when compared to ara-C, no activity was reported with regard to their effectiveness on solid tumors.
Another approach to forming ara-C prodrugs is reported by Ichikawa, H. et al. in Drug Design and Discovery, 1993, Vol. 10, pp. 343-353. In this case, chitosan (a biodegradable polymer with repeating pendant groups) is linked to ara-C via a glutaric anhydride spacer. The reactions, however, are low yield and, like the PLGA and PHEG conjugates of ara-C described above, the chitosan-spacer-ara-C conjugates suffer from the same unpredictable loading on the pendant groups and subsequently hydrolysis of the ara-C.
Thus, in spite of the advances already made with ara-C, there continues to be a need for further improvements. The present invention addresses this need.
In one aspect of the invention, compounds of Formula (I) are provided: 
wherein:
G is a linear or branched terminally functionalized polymer residue;
Y1 is O, S, or NR1;
M is selected from either X or Q;
wherein X is an electron withdrawing group and Q is a moiety containing a free electron pair positioned three to six atoms from C(xe2x95x90Y1); 
R1-5 are independently selected from the group consisting of hydrogen, C1-6 alkyls, C3-12 branched alkyls, C3-8 cycloalkyls, C1-6 substituted alkyls, C3-8 substituted cycloalkyls, aryls, substituted aryls, aralkyls, C1-6 heteroalkyls, substituted C1-6 heteroalkyls;
R6 is OR7 or N3, NH2, NO2 or CN, where R7 is selected from the same group as that which defines R1-5;
R8-9 are independently selected from the group consisting of hydrogen, fluoro, chloro, bromo, iodo, or R6; and
a and n are each independently zero or a positive integer, preferably from about 1 to about 5.
In preferred embodiments, Y1 is O, G is a poly(ethylene glycol) residue, and M is either NH or oxygen.
For purposes of the present invention, the term xe2x80x9cresiduexe2x80x9d shall be understood to mean that portion of a biologically active compound which remains after the biologically active compound has undergone a substitution reaction in which the prodrug carrier portion has been attached.
For purposes of the present invention, the term xe2x80x9calkylxe2x80x9d shall be understood to include straight, branched, substituted, e.g. halo-, alkoxy-, and nitro-C1-12 alkyls, C3-8 cycloalkyls or substituted cycloalkyls, etc.
For purposes of the present invention, the term xe2x80x9csubstitutedxe2x80x9d shall be understood to include adding or replacing one or more atoms contained within a functional group of compound with one or more different atoms. The term xe2x80x9csufficient amountsxe2x80x9d for purposes of the present invention shall mean an amount which achieves a therapeutic effect as such effect is understood by those of ordinary skill in the art.
One of the chief advantages of the polymer conjugates of the present invention is the fact that the ara-C residues are only found on the termini of the polymer. The uniform polymeric conjugates are thus easy to analyze and are highly reproducible. The rate of hydrolysis is also predictable and reproducible from batch to batch. Still, a further advantage is that in certain preferred embodiments, in which the polymer portion has a molecular weight of from about 20 to about 50 kDa, the conjugates are believed to passively target tumors and thus enhance the effectiveness of ara-C and related compounds on solid tumors. While applicants are not bound by theory, it is believed that tumor proteases, alone and/or in combination with peptidases, cleave the aromatic amide or carbamate bonds, thus freeing the parent active agent within the tumor. Methods of making and using the compounds and conjugates described herein are also provided.